SAN FRANCISCO: US online giant Amazon says it added three million customers to its Prime subscription service worldwide in the third week of December as it capped a record holiday season. Prime, which costs $99 a year in the United States, gives members fast shipping options as well as access to its streaming video, music and other services. Prime is seen as a way to get more customers into the Amazon ecosystem, including its video service that competes with rivals like Netflix and is investing heavily in its own programming.